When I first discovered that swimming in the Top End seas was a rare activity, I was dismayed. I have spent the past five years living and diving from Indonesian and Mozambican beaches almost every day. I wondered how I’d cope without my daily dose of vitamin sea and sealife!

I’d heard about the Top End’s infamous crocodiles, stingers and limited visibility. So it was with some trepidation that I packed my fins, mask and snorkel for my first adventure into local waters. When we arrived at a blue hole near the Vernon Islands off Gunn Point I was delighted to find clear waters and none of the aforementioned crocodiles or stingers. Appreciating my good fortune with conditions I couldn’t wait to jump in and explore.

Blue holes traditionally are large marine sinkholes that open to the surface and are ringed by a bank of limestone or coral. While our blue hole isn’t a sinkhole, it is surrounded by sandy banks and exposed reef at low tide, creating a massive bath tub.The water was also nice and hot like a relaxing bath.

From the surface we could see shelves of colourful coral tables and staghorn forests descending and disappearing into the depths. Fishlife appeared sparse - that was until we put our faces in the water! I was shocked to see schools of yellowtail fusiliers (Caesio teres), golden trevally (Gnathanodon speciosus) and trout zipping past beneath the thermocline. Diving down revealed cooler waters and even more species: angel fish, cardinals, parrotfish and batfish. I swam across the hole thinking I had spotted a beautiful boulder coral - but it was actually a cruising hawksbill turtle! Upon the edge of visibility some juvenile black tip reef sharks (Carcharhinus melanopterus) dashed by. To my delight, I also spotted a new species of shark for me, the vulnerable tawny nurse shark, which was lying upon the sandy bottom where it can change colour between grey and sandy brown depending on its surroundings. Beautiful!

But the real showstopper was the natural oceanic-waterfall…

As we motored towards the edge of the blue hole I couldn’t quite grasp what I was seeing. The ocean on the other side of the divide looked lower, as though we were sitting above it. Surely this is impossible as the ocean is one level, I thought. The water from our blue hole enclosure was rushing and bubbling down a limestone-based ‘horizontal’ waterfall.

Coral reefs accumulate as generations of coral build upon the “bones of their ancestors.”(1) Given the right conditions, rising sea levels enable coral reefs to grow vertically. (2) Maybe this is how our blue hole formed and the explanation behind the elevation difference...

We visited this area to investigate what marine wonders may be lost if the government were to develop the proposed “heavy industry port” at Glyde Point. (3) If this development does go ahead our unique blue holes full of fish and coral reef gardens - just like this one at Gunn Point - would be damaged by dredge spoil, pollution and sedimentation. These marine areas are beautiful; they are healthy; they are inspiring and we must ensure they stay this way for years to come.

References

(1) www.skepticalscience.com/Coral_atolls_and_rising_sea_levels_that_sinking_feeling.html

(2) sydney.edu.au/news/84.html?newsstoryid=14522

(3) www.abc.net.au/news/2014-10-24/brewing-fight-over-port-expansion-in-darwin/5840818